Endoscopes generally include a series of lenses used to convey an image of the scene to be viewed from a distal end of the endoscope to a proximal end, where the image may be observed by the user, captured by an image sensor, recorded by a video camera, and/or processed by electronic means. (Throughout this application, the “distal” end of the endoscope will refer to the objective end, while the “proximal” end will refer to the image end.) It is well known in the art that objective lens systems in endoscopes often include two lens groups separated by an aperture diaphragm. As disclosed in U.S. Pat. No. 5,005,957 to Kanamori et al. and U.S. Pat. No. 6,618,207 to Lei, the first lens group has an overall negative refractive power while the second lens group has an overall positive refractive power. This well-known optical design form of endoscope objective lens systems is referred to as reverse telephoto.
An endoscope objective lens arrangement 10 of the reverse telephoto type can be seen in FIG. 1. Lens arrangement 10 has a first lens group 11 having an overall negative refractive power nearest the distal end 14 of the endoscope, a second lens group 12 having an overall positive refractive power nearer to the proximal end 18 of the endoscope, an aperture diaphragm 13 situated between said first lens group 11 and said second lens group 12, and an image capture device 25. First lens group 11 contains lens element 16 having a negative refractive power. Lens element 16 is situated so that its concave surface 19 faces the aperture diaphragm 13 and the proximal end of the endoscope 18. First lens group 11 also contains cover glass 15, which acts as a barrier between the optical system and the environment external to the endoscope.
Endoscopes are intended to be used in minimally invasive surgical techniques, such as laparoscopy, hysteroscopy, and colonoscopy. As a result, it is desirable to reduce the size of the endoscope as much as possible in order to limit the stress on a surgical patient's body tissues. The most important dimension of the endoscope for these purposes is its outside diameter, which is substantially limited by the diameter of the lenses inside the endoscope. Therefore, reducing the size of the endoscope is best achieved by reducing the diameter of the lenses.
One advantage of the reverse telephoto configuration is that it allows for a relatively large field of view for an endoscope of small diameter. To obtain a large field of view, reverse telephoto designs generally employ one or more negative refractive power lens surfaces in the first lens group. Negative refractive power lens surfaces having their concave surfaces facing the aperture diaphragm usually provide most of the negative power of the first lens group which has an overall negative power. In many endoscopes, the first lens group is a single negative power lens with its concave surface facing the aperture diaphragm.
As stated above, because of the intended application of endoscopes, it is desirable to reduce the diameter of the lenses as much as possible. Unfortunately, as a lens's diameter is reduced, its refractive power must be increased in order to maintain the size of the lens's field of view. As the diameter is reduced and the lens gets smaller, it becomes increasingly difficult to manufacture the curved surface of the lens. As a result, it is difficult and sometimes impossible to create a lens with a sufficiently small radius of curvature. Thus, as the lens diameter requirements become more stringent and call for a smaller lens, it becomes more difficult and sometimes impossible to manufacture the lens with sufficient power due to the difficulty of creating a small radius of curvature.
Endoscope designers are thus faced with a choice when utilizing the conventional approach to endoscope design. On one hand, the performance of the optical system may be maintained at the expense of any significant reduction of the endoscope's size. On the other hand, the endoscope may be reduced in size at the expense of optical performance. It has been discovered that simply scaling a conventional endoscope lens configuration down to a smaller size is not possible. The overall size of the endoscope and the performance of the optical system are limited by the manufacturability of the lenses.
U.S. Pat. No. 6,134,056 to Nakamuka is directed to designs of objective lens systems for endoscopes that attempt to address these problems. Nakamuka discloses objective lens systems including a negative first lens in the first lens group having a concave surface facing the distal end of the endoscope. However, each of these designs is limited by the following condition:2.0<f3/f<5.0Where f3 is the focal length of the third lens in the system and f is the overall focal length of the system. Nakamuka imposes this restriction for the stated reasons that a value of f3/f of less than 2.0 would result in a larger-than-desired diameter of the third lens and a value of f3/f of more than 5 would introduce undesirable color shading into the image. However, it has been found that designing f3/f to less than 2.0 does not require a larger diameter in the third lens while at the same time minimizing color shading in the image.
Using conventional endoscope objective lens configurations has limited the creation of miniature endoscopes which would be of great value in the field. What is desired, then, is an endoscope lens configuration design which minimizes the diameter of the lenses without sacrificing the optical performance of the system to an extent that use of the endoscope becomes impractical or undesirable. It is also desirable that this design be simple and inexpensive to implement.